Excess air factor control of diesel engine

ABSTRACT

A diesel engine ( 1 ) provided with a NOx catalyst ( 28 A) and a diesel particulate filter ( 28 B) performs lean burn operation during normal running, rich burn operation during regeneration of the NOx catalyst ( 28 A), operation under the stoichiometric air-fuel ratio during desulphating of the NOx catalyst ( 28 A), and operation under a slightly lean air-fuel ratio to regenerate the filter after desulphating of the NOx catalyst ( 28 A). When the lean burn operation is applied, a controller ( 31 ) first controls the fuel injection amount, and controls an air supply amount based on the fuel injection amount. When rich burn operation is applied, the controller ( 31 ) first controls the air supply amount and controls the fuel injection amount based on the air supply amount. Due to this control, the response of the excess air factor control is enhanced while preventing torque fluctuation accompanying the variation of the target excess air factor.

FIELD OF THE INVENTION

This invention relates to control of the excess air factor of a dieselengine.

BACKGROUND OF THE INVENTION

Exhaust gas purification techniques in diesel engines for vehicles aredivided roughly into two categories, one for improving combustion of theair-fuel mixture so as to reduce generation of noxious substances, andthe other one for purifying the noxious substance contained in theexhaust gas.

As to the former techniques, Tokkai Hei 6-346763 published by theJapanese Patent Office in 1994 discloses performing exhaust gasrecirculation (EGR) and making the ignition lag, i.e., the lag betweenthe start of fuel injection and the start of combustion, considerablylonger.

EGR brings about a decline of combustion temperature and reduces thegeneration of nitrogen oxides (NOx) as a result.

If the ignition lag is made considerably longer, the engine heatgeneration pattern becomes single stage and mixing of air and fuel ispromoted, so the combustion type will be pre-mixing combustion. Thepre-mixing combustion has the effect of suppressing the generation ofparticulate matter (PM).

As for the latter techniques, Tokkai Hei 6-272541 and Tokkai Hei6-159037 published by the Japanese Patent Office in 1994 disclose a NOxtrap catalyst and a diesel particulate filter (DPF).

The NOx trap catalyst traps the NOx in the exhaust gas when the dieselengine performs combustion at a lean air-fuel ratio, and reduces trappedNOx by reducing agents such as hydrocarbons (HC) and carbon monoxide(CO) in the exhaust gas when the engine performs combustion at astoichiometric air-fuel ratio or a rich air-fuel ratio.

By periodically performing the combustion at the stoichiometric or richair-fuel ratio, therefore, the NOx is purified and the NOx trap functionof the catalyst is regenerated.

The diesel particulate filter traps the PM contained in the exhaust gasof the diesel engine, and when the trapped PM reaches a fixed quantity,the engine burns the trapped PM at the stoichiometric or slightly leanair-fuel ratio so as to purify the PM and regenerate the filter.

SUMMARY OF THE INVENTION

The diesel engine provided with such an exhaust gas purifying device isusually operated under a lean air-fuel ratio at which the excess airfactor is 1.4 or more.

When there is a need to regenerate the NOx trap catalyst or dieselparticulate filter, the engine is operated at a rich air-fuel ratiowhere the excess air factor is less than unity or at the stoichiometricair-fuel ratio at which the excess air factor is equal to unity.

As a control for realizing a target excess air factor, U.S. Pat. No.6,247,311 discloses an excess air factor control system in which atarget fuel injection amount is first determined according to thedepression amount of the accelerator pedal of the vehicle, and theengine rotation speed.

The target intake air amount is then calculated from the target fuelinjection amount and the target excess air factor. The system controlsthe opening of the intake throttle, turbocharging pressure of theturbocharger, and the opening of the exhaust gas recirculation valve sothat the target intake air amount is realized.

As fuel is an incompressible fluid, and the fuel injector is installedin the engine combustion chamber or its vicinity in the diesel engine,the control of fuel injection amount is not accompanied by a time delay.

On the other hand, the turbocharger and an intake throttle both of whichadjust the intake air amount are installed at a position distant fromthe combustion chamber of the engine, and as air is a compressiblefluid, control of the intake air amount is accompanied by a large timedelay.

Therefore, when shifting the target excess air factor from 1.4 or moreto 1.0 or a lower value for regeneration of the NOx trap catalyst ordiesel particulate filter, the most responsive way of control to achievethe new target excess air factor is to increase the fuel injectionamount.

However, if the fuel injection amount is increased when the targetexcess air factor is larger than unity, i.e., at a lean air-fuel ratio,the engine output torque will increase.

It is not desirable that the engine output torque increases every timethe NOx trap catalyst or diesel particulate matter filter isregenerated.

There is also a different approach to attain the new target excess airfactor in a short time but without increasing the engine output torque.This is the delay compensation control of the intake air amount. In thiscontrol, the delay in the intake air amount control is previouslyassessed and compensated by a delay compensation process based on theassessed delay.

However, the delay of intake air amount varies largely according to theengine running conditions, and it is difficult to compensate the delaywith sufficient accuracy under various engine operating conditions.

It is therefore an object of this invention to increase the accuracy andresponse of the excess air factor control while preventing the change inthe engine output torque.

In order to achieve the above object, this invention provides an excessair factor control device for such a diesel engine that burns a mixtureof air supplied by an air supply mechanism and fuel supplied by a fuelsupply mechanism.

The device comprises a sensor which detects a running state of thediesel engine; and a programmable controller. The programmablecontroller is programmed to set a target excess air factor of themixture based on the running state, control an air supply amount of theair supply mechanism to a target air supply amount calculated from apredetermined fuel supply amount and the target excess air factor whenthe target excess air factor is larger than a value equivalent to astoichiometric air-fuel ratio, and control a fuel supply amount of thefuel supply mechanism to a target fuel supply amount calculated from apredetermined air supply amount and the target excess air factor whenthe target excess air factor is smaller than the value equivalent to thestoichiometric air-fuel ratio.

This invention also provides an excess air factor control method for thediesel engine, comprising detecting a running state of the dieselengine, setting a target excess air factor of the mixture based on therunning state, controlling an air supply amount of the air supplymechanism to a target air supply amount calculated from a predeterminedfuel supply amount and the target excess air factor when the targetexcess air factor is larger than a value equivalent to a stoichiometricair-fuel ratio, and controlling a fuel supply amount of the fuel supplymechanism to a target fuel supply amount calculated from a predeterminedair supply amount and the target excess air factor when the targetexcess air factor is smaller than the value equivalent to thestoichiometric air-fuel ratio.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an excess air factor control device ofa diesel engine according to this invention.

FIG. 2 is a diagram describing an operating range of the diesel engine.

FIG. 3 is a diagram describing a relation between engine operationconditions, combustion type, and air-fuel ratio.

FIG. 4 is a block diagram describing the function of a programmablecontroller according to this invention.

FIG. 5 is a block diagram describing a process for calculating a targetfresh air amount, a target fuel injection amount and a target fuelinjection timing performed by the controller.

FIGS. 6A–6G are timing charts describing a fuel-based control of anexcess air factor.

FIGS. 7A–7G are timing charts describing an air-based control of theexcess air factor.

FIGS. 8A–8E are timing charts describing a fuel-based control of anengine output torque.

FIGS. 9A–9E are timing charts describing an air-based control of theengine output torque.

FIGS. 10A–10E are timing charts describing a change of the engine outputtorque when switching from lean burn operation to rich burn operationunder the excess air factor control according to this invention.

FIG. 11 is a diagram showing the characteristics of a map of a targetfresh air basic value amount tQacb stored by the controller.

FIG. 12 is a diagram showing the characteristics of a map of an excessair factor conversion coefficient kQaclm stored by the controller.

FIG. 13 is a diagram showing the characteristics of a map of a targetengine output torque Ttrq stored by the controller.

FIG. 14 is a diagram showing the characteristics of a map of a targetexcess air factor basic value Tlamb0 stored by the controller.

FIG. 15 is a diagram showing the characteristics of a map of a targetEGR rate Megr stored by the controller.

FIG. 16 is a diagram showing the characteristics of a map of a targetfuel injection amount TQfF in fuel-based control stored by thecontroller.

FIG. 17 is a diagram showing the characteristics of a map of a targetfuel injection timing basic value MIT stored by the controller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a multi-cylinder diesel engine 1 isprovided with an exhaust passage 2 and an intake passage 3.

The intake passage 3 is connected to an intake port 3C provided for eachcylinder via an intake manifold 3B which branches off from a collector3A.

The exhaust gas passage 2 is connected to an exhaust port 2C providedfor each cylinder via an exhaust manifold 2B.

Air from a compressor 23 of a variable capacity turbocharger 21 issupplied to the intake passage 3.

The variable capacity turbocharger 21 drives the compressor 23 byrotation of an exhaust gas turbine 22 installed in the exhaust passage2.

The exhaust gas turbine 22 is provided with a variable nozzle 24 drivenby an actuator 25 at the inlet of a scroll.

In the low rotation speed region of the diesel engine 1, the variablenozzle 24 increases the flow velocity of the exhaust gas which flowsinto the exhaust gas turbine 22 by narrowing the opening, and in thehigh rotation speed region, it is opened fully to reduce the inflowresistance of exhaust gas to the exhaust gas turbine 22.

The actuator 25 comprises a diaphragm actuator 26 which drives thevariable nozzle 24 according to a supply pressure, and a pressurecontrol valve 27 which supplies pressure to the diaphragm actuator 26.

The pressure control valve 27 supplies the pressure according to apressure signal which a programmable controller 31 outputs to thediaphragm actuator 26.

An intake throttle 18 which adjusts the intake air amount of the dieselengine 1 is installed upstream of the collector 3A of the intake passage3.

The intake throttle 18 is operated via an actuator 19 which responds toa drive signal from the controller 31

A swirl control valve 8 operated by an actuator 38 is installed in theintake port 3C. The velocity of intake air is increased by closing theswirl control valve 8 when the intake air amount of the diesel engine 1is small. A swirl is formed inside each cylinder, and mixing of air andfuel is thereby promoted.

The actuator 38 comprises a diaphragm actuator 36 which drives the swirlcontrol valve 8 according to a supplied pressure and a pressure controlvalve 37 which supplies pressure to the diaphragm actuator 36 inresponse to a pressure signal supplied from the controller 31.

A fuel injector 17 provided in each cylinder injects fuel into the airaspirated by each cylinder from the intake passage 3 and a mixture ofair and fuel is generated in each cylinder.

The diesel engine 1 burns this air-fuel mixture by compression ignitionto generate motive force.

Fuel from a fuel supply device 10 is supplied to the fuel injector 17.The fuel supply device 10 is provided with a supply pump 14 and acommon-rail 16.

The supply pump 14 pressurizes the fuel in a fuel tank, not shown, andsupplies it to the common-rail 16.

The common-rail 16 is provided with a pressurizing chamber, and suppliesfuel to each fuel injector 17 under a fixed fuel pressure. The fuelinjector 17 opens according to a fuel injection signal from thecontroller 31, and injects fuel into each cylinder.

A fuel injection signal comprises pulse signals, and the pulse widthcorresponds to the fuel injection amount. The timing of the appearanceof a pulse signal corresponds to the fuel injection timing.

The fuel injector 17 therefore constitutes a fuel supply mechanism ofthe diesel engine 1.

The combustion gas of the air-fuel mixture is discharged into theatmosphere through the exhaust passage 2 from the exhaust port 2C andexhaust manifold 2B.

A part of the exhaust gas in the exhaust passage 2 is recirculated intothe collector 3A via an exhaust gas recirculation (EGR) passage 4. Anexhaust gas recirculation (EGR) valve 6 for adjusting the exhaust gasflowrate of the EGR passage 4 is provided in the EGR passage 4. The EGRvalve 6 is a diaphragm-type valve which responds to a drive signal fromthe controller 31.

An exhaust gas purification device 28 is installed downstream of theexhaust gas turbine 22 of the exhaust passage 3. The exhaustpurification device 28 comprises a NOx trap catalyst 28A and a dieselparticulate matter filter (DPF) 28B accommodated in a single casing.

The NOx trap catalyst 28A traps the NOx contained in the exhaust gasunder a lean air-fuel ratio, and reduces NOx to harmless nitrogen andharmless oxygen by HC and CO in the exhaust gas generated under a richair-fuel ratio or a stoichiometric air-fuel ratio. Due to the reductionof the trapped NOx, the NOx trap function of NOx trap catalyst 28A isregenerated.

The DPF28B traps the particulate matter (PM) in the exhaust gas. Thetrapped PM is burned by the operation of the diesel engine 1 at aslightly lean air-fuel ration with respect to the stoichiometricair-fuel ratio. The PM trap function of the DPF28B is regenerated byburning the trapped PM.

The diesel engine 1 lowers the oxygen concentration of the intake air byperforming EGR, and reduces the NOx generation amount by reducing thecombustion temperature of the air-fuel mixture.

Also, the PM generation amount is reduced by considerably lengtheningthe ignition lag and realizing pre-mixing combustion by a single stageheat generation pattern.

In order to realize the pre-mixing combustion, it is necessary tomaintain the combustion temperature and ignition lag within apredetermined range.

In the high load region of the diesel engine 1 with a high EGR gastemperature, or the high rotation speed region with a short combustionperiod, pre-mixing combustion is impossible.

Therefore, in the region in which pre-mixing combustion is impossible,so-called diffusive combustion is performed where combustion isperformed while mixing fuel and air.

Referring to FIG. 2, pre-mixing combustion is performed in a pre-mixingcombustion region A specified by an engine rotation speed Ne and engineoutput torque, and diffusive combustion is performed in other regions Band C.

Further, in the Region A and Region B, a lean air-fuel ratio is applied,and a rich air-fuel ratio is applied in Region C.

In operation under a lean air-fuel ratio, NOx is trapped by the NOx trapcatalyst 28A. If this trapped amount reaches a fixed quantity, operationof the diesel engine 1 will be performed under a rich air-fuel ratio inorder to reduce the trapped NOx and regenerate the trap capability ofthe NOx trap catalyst 28A.

The NOx trap catalyst 28A is poisoned by the very small quantity of SOxcontained in the exhaust gas. Therefore, when the SOx deposition amountof the NOx trap catalyst 28A reaches a tolerance limit, operation of adiesel engine 1 is performed near the stoichiometric air-fuel ratio inorder to raise the exhaust gas temperature to cause the deposited SOx tobe released from the NOx trap catalyst 28A. This process is referred toas desulphating.

When the PM trapped by the diesel particulate filter 28B reaches themaximum permissible quantity, the exhaust gas temperature is raised toaround 300 degrees C., the PM is burned, and the diesel particulatefilter 28B is regenerated. A slightly lean air-fuel ratio with respectto the stoichiometric air-fuel ratio is applied in this operation.

The control of these air-fuel ratios is performed by the controller 31by operating the fuel injector 17, the variable nozzle 24 and the EGRvalve 6.

If only the turbocharger 21 is operated in the low load region near idlerotation, combustion at the desired rich air-fuel ratio orstoichiometric air-fuel ratio is not realized easily. Hence, in the lowload region near idle rotation, a rich air-fuel ratio or thestoichiometric air-fuel ratio is achieved by throttling the intakethrottle 18 to suppress the intake air amount. Instead of the intakethrottle 18, an exhaust throttle may be provided.

The EGR valve 6 varies the exhaust gas recirculation amount, and variesthe proportion of the fresh air amount aspirated by the diesel engine 1in the total intake gas amount thereof.

The above variable nozzle 24, intake throttle 18, and EGR valve 6constitute an air supply mechanism in this diesel engine 1.

Referring now to FIG. 3, the operation of the diesel engine 1 forreduction of NOx, desulphating and burning PM is performed only whenvarious conditions are satisfied. In the following description, theengine operations for these specific purposes are generically termedspecific operations.

On the other hand, the conditions under which the diesel engine 1carries out pre-mixing combustion or diffusive combustion under a leanair-fuel ratio are referred to as normal operation. Operation bycombustion of an air-fuel mixture of a lean air-fuel ratio in the dieselengine 1 is referred to as a lean burn operation, and combustion of anair-fuel mixture of a rich air-fuel ratio is referred to as a rich burnoperation.

If reduction of NOx is needed during normal operation, NOx reduction bya rich burn operation will be performed as a specific operation, and theengine will return to normal operation by lean burn operation aftercompletion of the operation. If desulphating is needed during normaloperation, desulphating by an engine operation with the stoichiometricair-fuel ratio will be performed as a specific operation, combustion ofPM by an engine operation with a slightly lean air-fuel ratio will thenbe performed as a specific operation, and the engine will return tonormal operation after completion of the operation.

In control of the excess air factor during lean burn operation,air-based control is applied, and during rich burn operation and anengine operation at the stoichiometric air-fuel ratio, fuel basedcontrol is applied.

Further, the fuel injection timing is also controlled to suit thecombustion type.

The above excess air factor and fuel injection control performed by thecontroller 31 are summarized in Table 1.

When there is a change-over to rich burn operation or stoichiometricair-fuel ratio operation from lean burn operation or vice-versa, thecontroller 31 performs delay processing on a target fuel injectiontiming basic value and a target excess air factor basic value.

Now, the difference between air-based control and fuel-based controlwill be described.

The target excess air factor and target EGR rate are given by thefollowing Equations (1) and (2). $\begin{matrix}{{Tlamb} = \frac{Gac}{{{Gfc} \cdot {BLAMB}}\#}} & (1) \\{\;{{EGRr} = {\frac{Gegr}{Gac} \cdot 100}}} & (2)\end{matrix}$

where, Tlamb=target excess air factor,

-   -   Gac=target cylinder intake air amount,    -   Gfc=target fuel injection amount,    -   EGRr=target EGR rate,    -   Gegr=target EGR amount, and    -   BLAMB#=stoichiometric air-fuel ratio=14.7.

Also in both air-based control and fuel-based control, the target excessair factor Tlamb and the target EGR rate EGRr are applied as controltarget values respectively.

In fuel-based control, the target fuel injection amount Gfc iscalculated based on an accelerator pedal depression amount APSrepresenting the engine load, and the engine rotation speed Ne.

The target cylinder intake air amount Gac is calculated by Equation (1)from the target fuel injection amount Gfc and the target excess airfactor Tlamb.

The target EGR rate EGRr and target cylinder intake air amount Gac arethen substituted into equation (2) to calculate the target exhaust gasrecirculation amount Gegr. This control process is basically the same asthe control method disclosed by the above-mentioned U.S. Pat. No.6,247,311.

On the other hand, in air-based control, the measured intake fresh airamount is considered as the target cylinder intake air amount Gac, andthe target fuel injection amount Gfc is calculated from the targetcylinder intake air amount Gac and target excess air factor.

The target cylinder intake air amount Gac and the target EGR rate EGRrare substituted into equation (2) to calculate the target EGR amountGegr.

To perform the above control, detection data are input to the controller31 from an accelerator pedal depression sensor 32 which detects thedepression amount APS of an accelerator pedal with which the vehicle isprovided, a crank angle sensor 33 which detects the rotation speed Neand a crank angle of the diesel engine 1, and an air flow meter 35 whichdetects a flowrate of the fresh air supplied from the compressor 23 tothe intake passage 3. The controller 31 comprises a microcomputerequipped with a central processing unit (CPU), read-only memory (ROM),random access memory (RAM) and I/O interface (I/O interface). It is alsopossible to construct the controller 31 from plural microcomputers.

Next, the functions of the controller 31 for performing theabove-mentioned control will be described, referring to FIG. 4.

The controller 31 is provided with a specific operation requestdetermination unit 41, a target torque setting unit 42, a target excessair factor basic value setting unit 43, a target EGR rate setting unit44, an intake fresh air amount calculation unit 45, a target valuesetting unit 46, a fuel injector control unit 47 an EGR valve controlunit 48, a turbocharger control unit 49 and an intake throttle controlunit 50.

These units are virtual units for describing the functions of thecontroller 31, and do not exist physically.

The specific operation request determination unit 41 determines the NOxtrap amount of the NOx catalyst 28A of the exhaust gas processor 28, anda sulphur poisoning amount based on the operating time of the dieselengine 1.

That is, when the cumulative operating time of the diesel engine 1 afterperforming NOx reduction reaches a predetermined time, it is determinedthat the NOx trap amount is saturated.

Similarly, when the cumulative operating time of the diesel engine 1after performing desulphating reaches a predetermined time, it isdetermined that the sulphur poisoning amount has reached the limit.

The specific operation request determination unit 41, based on thesedetermination results, selects the operation type of the diesel engine 1from among the normal operation, NOx reduction, desulphating andregeneration of the DPF 28B.

Herein, the regeneration of the DPF 28B is always performed afterdesulphating.

The target engine torque setting unit 42 calculates a target outputtorque Ttrq of the diesel engine 1 by looking up a map having thecharacteristics shown in FIG. 13 based on the accelerator pedaldepression amount APS and the engine rotation speed Ne.

The target excess air factor basic value setting unit 43 sets a targetexcess air factor basic value Tlamb0 by the following process.

(1) During normal operation, the target excess air factor basic valueTlamb0 is calculated by looking up a map having the characteristicsshown in FIG. 14 from the target engine output torque Ttrq and theengine rotation speed Ne. Tlamb0 calculated here is a larger value than1.4.

(2) During a specific operation, the target excess air factor basicvalue Tlamb0 is set as a value which is predefined according to the typeof the operation.

Specifically in the NOx reduction, Tlamb0 is set to a value smaller than1.0.

In the desulphating process, Tlamb0 is set to 1.0.

In the regeneration of DPF 28B, Tlamb0 is set to a slightly larger valuethan 1.0.

The target EGR rate setting unit 44 calculates the target EGR rate Megrby looking up a map having the characteristics shown in FIG. 15 from thetarget engine output torque Ttrq and the engine rotation speed Ne.

The target EGR rate Megr is held at the same value even when operationof the diesel engine 1 changes over from normal operation to a specificoperation as long as the target engine output torque Ttrq and enginerotation speed Ne remain unchanged. The intake fresh air mountcalculation unit 45 calculates the intake fresh air amount Qac perstroke of a cylinder by adding processing for dead time and a firstorder delay to the air flowrate detected by the air flow meter 35.

The method of calculating the fresh air intake amount Qac from thedetection flowrate of the air flow meter 35 is known from U.S. Pat. No.5,964,820.

The target value setting unit 46 sets the target fresh air amount TQa,target fuel injection amount TQf and target fuel injection timing MITFso that the target excess air factor basic value Tlamb0 set by thetarget excess air factor basic value setting unit 43 may be realized.

This function of the target value setting unit 46 will now be describedin detail, referring to FIG. 5.

FIG. 5 shows the internal construction of the target value setting unit46.

The units 62–74 shown in the drawing are also virtual units fordescribing the function of the target value setting unit 46.

The delay processing unit 73 weight averages the target excess airfactor basic value Tlamb0.

When a control flag which will be described later is zero, the switch 74sets the target excess air factor Tlamb equal to the target excess airfactor basic value Tlamb0.

When the control flag is unity, the weighted average value calculated bythe delay processing unit 73 is set as the target excess air factorTlamb.

The conversion unit 62 and the target fuel injection amount calculationunit 63 calculate a target fresh air amount TQaA and target fuelinjection amount TQfA in air-based excess air factor control.

The conversion unit 62 converts the target excess air factor Tlamb intothe target fresh air amount TQaA taking account of the exhaust gasrecirculation amount. For this purpose, the conversion unit 62 firstcalculates the target fresh air amount basic value tQacb by looking up amap having the characteristics shown in FIG. 11 from the target enginetorque Ttrq set by the target engine torque setting unit 42, and theengine rotation speed Ne.

A conversion coefficient kQaclm used for the conversion of the excessair factor to the intake fresh air amount is also calculated from thetarget excess air factor Tlamb and the engine rotation speed Ne bylooking up a map having the characteristics shown in FIG. 12.

The target fresh air amount TQaA in air-based control is calculated bythe next Equation (3) using these values. $\begin{matrix}\begin{matrix}{{TQaA} = {\frac{tQacb}{1 + {Megr}} \cdot {kQaclm}}} \\{{where},{\frac{1}{1 + {Megr}} = {{correction}\mspace{14mu}{coefficient}\mspace{14mu}{of}\mspace{14mu}{EGR}\mspace{14mu}{{amount}.}}}}\end{matrix} & (3)\end{matrix}$

The target fuel injection amount calculation unit 63 calculates thetarget fuel injection amount TQfA in air-based control by the nextEquation (4) from the target excess air factor Talmb and intake freshair amount Qac. $\begin{matrix}{{TQfA} = \frac{Qac}{{{Tlamb} \cdot {BLAMB}}\#}} & (4)\end{matrix}$

On the other hand, the target fuel injection amount calculation unit 64and the target intake fresh air amount calculation unit 65 calculate thetarget fuel injection amount TQfF and target fresh air amount TQaF infuel -based control.

The target fuel injection amount calculation unit 64 calculates thetarget fuel injection amount TQfF of fuel-based control by looking up amap having the characteristics shown in FIG. 16 from the target enginetorque Ttrq and the engine rotation speed Ne.

The target intake fresh air amount calculation unit 65 calculates thetarget fresh air amount TQaF in fuel-based control by the next Equation(5) from the target fuel injection amount TQfF and the target excess airfactor Tlamb.TQaF=Tlamb·TQfF·BLAmB#  (5)

The combustion determination unit 66 and switches 67, 68 selectivelyapply air-based control or fuel-based control according to thedetermination results as to which of the lean burn operation, rich burnoperation, or stoichiometric air-fuel ratio operation is to be applied.

The combustion determination unit 66 determines as to which of lean burnoperation, rich burn operation, or the stoichiometric air-fuel ratiooperation should be applied from the target excess air factor Tlamb andthe determination result of the specific operation request determinationunit 41.

When lean burn operation should be applied, the control flag is set tounity.

When rich burn operation or stoichiometric air-fuel ratio operationshould be applied, the control flag is set to zero.

Referring again to Table 1 here, the control flag will be unity duringregeneration of the DPF 28B and during normal operation.

The control flag is set to zero during NOx reduction and desulphating.

Switches 67, 68 respond to the control flag. When the control flag isunity, the switch 67 outputs the target fresh air amount TQaA inair-based control as the target fresh air amount TQa, and when thecontrol flag is zero, the target fresh air amount TQaF in fuel-basedcontrol is output as the target fresh air amount TQa.

When the control flag is unity, the switch 68 outputs the target fuelinjection amount TQfA in air-based control as the target fuel injectionamount TQf, and when the control flag is zero, the target fuel injectionamount TQfF in fuel-based control is output as the target fuel injectionamount TQf.

The target fuel injection timing basic value setting unit 69 sets thetarget fuel injection timing basic value MIT as follows according to thetarget excess air factor basic value Tlamb0 and the determination resultof the specific operation request determination unit 41.

-   -   (1) When the determination result of the specific operation        request determination unit 41 is normal operation, the target        fuel injection timing basic value MIT is calculated by looking        up a map having the characteristics shown in FIG. 17 from the        target engine torque Ttrq and the engine rotation speed Ne.    -   (2) When the determination result of the specific operation        request determination unit 41 is regeneration of the DPF 28B,        the target fuel injection timing basic value MIT is selectively        set according to the operating range A or B shown in FIG. 2.    -   (3) When the determination result of the specific operation        request determination unit 41 is NOx reduction or desulphating,        a value obtained by advancing 10 to 15 degrees from normal        operation is set as the target fuel injection timing basic value        MIT.

In the following cases where the air-fuel ratio varies, the transientstate determination unit 70 sets a transient state flag for performingdelay processing on the target fuel injection timing basic value MIT andthe target excess air factor basic value Tlamb0, i.e.,

(1) change-over from normal operation to NOx reduction operation,

(2) change-over from NOx reduction operation to normal operation,

(3) change-over from normal operation to desulphating operation, and

(4) change-over from regeneration of DPF 28B to normal operation.

The transient state determination unit 70 determines whether the presentoperation conditions correspond to the transient conditions of theabove-mentioned cases (1)–(4) based on the control flag output by thecombustion determination unit 66.

When the conditions correspond to one of the transient conditions, itsets the transient state flag to unity over a fixed period.

When the conditions do not correspond to one of the transientconditions, it sets the transient state flag to zero.

The delay processing unit 71 weight averages the target injection timingbasic value MIT.

When the transient state flag is zero, the switch 72 sets the targetfuel injection timing MITF equal to the target injection timing basicvalue MIT calculated by the target fuel injection timing basic valuesetting unit 69.

When the transient state flag is unity, the switch 72 sets the targetfuel injection timing MITF equal to the value calculated by the delayprocessing unit 71.

In this way, the target value calculation unit 46 calculates the targetfuel injection amount TQf and the target fuel injection timing MITF.

Referring again to FIG. 4, the fuel injector control unit 47 outputs afuel injection signal which has a pulse width according to the targetfuel injection amount TQf to the fuel injector 17 at a timing whichcoincides with the target fuel injection timing MITF.

The EGR valve control unit 48 calculates a target valve opening based onthe target fresh air amount TQa and target EGR rate Megr set by thetarget EGR rate setting unit 44, converts the target valve opening intoa duty signal, and outputs it to the EGR valve 6.

The turbocharger control unit 49 calculates the target opening of thevariable nozzle 24 based on the target fresh air amount TQa and targetEGR rate Megr, and outputs a corresponding signal to the pressurecontrol valve 27.

The intake throttle control unit 50 calculates the target opening of theintake throttle 8 based on the target fresh air amount TQa and targetEGR rate Megr, and outputs a corresponding signal to the pressurecontrol valve 37.

Next, the effect of the above control performed by the controller 31 onthe excess air factor and engine output torque will be describedreferring to FIGS. 6A–6G, 7A–7G, 8A–8E, 9A–9E, and 10A–10E.

FIGS. 6A–6G shows the variation of the excess air factor underfuel-based control, and FIGS. 7A–7G show the variation of the excess airfactor under air-based control.

In order to simplify the explanation here, the target EGR rate Megr isset constant and it is assumed that delay processing is omitted.

If the specific operation request determination unit 41 determines NOxreduction at the time t1 of FIG. 6A in the normal operation of thediesel engine 1, the required excess air factor varies from lean torich. Accordingly, the target excess air factor basic value setting unit43 immediately changes the target excess air factor basic value Tlamb0from lean to rich. As a result, the target excess air factor Tlamb isvaried from lean to rich, as shown in FIG. 6B.

On the other hand, in the target value calculation unit 46, the targetfuel injection amount TQfF of the fuel-based control calculated by thetarget fuel injection amount calculation unit 64 is set as the targetfuel injection amount TQf. Also, the target fresh air amount TQaF in thefuel-based control calculated by the target intake fresh air amountcalculation unit 65 based on TQfF is set as the target fresh air amountTQa.

As there is no response delay in fuel injection amount control, the fuelinjection amount increases immediately, but since control of the intakefresh air amount is accompanied by a time delay, the actual intake freshair amount shows a first order delay relative to change of the targetfresh air amount TQa, as shown by the dashed line of FIG. 6D.

Consequently, a real excess air factor also shows a change with a firstorder delay, as shown by the dashed line of FIG. 6G, and a considerabletime is required until the target excess air factor is realized.

By adding a delay compensation to the target fuel injection amount T

as shown by the dashed line of FIG. 6E, the excess air factor may varywith high response as shown by the straight line of FIG. 6G. However, asthe delay characteristics of the intake air amount largely change withthe operation conditions, it is difficult to compensate the delay undervarious operating conditions with sufficient accuracy.

On the other hand, if the required excess air factor varies from lean torich also in air-based control as shown in FIG. 7A, the target excessair factor Tlamb is varied from lean to rich immediately, as shown inFIG. 7B.

In the target value calculation unit 46, the conversion unit 62calculates the target fresh air amount basic value tQacb.

The target fuel injection amount calculation unit 63 calculates thetarget fuel injection amount TQfA in fuel-based control relative tovariation of the target fresh air amount basic value tQacb, targetexcess air factor Tlamb and fresh air intake amount Qac, and sets thetarget fuel injection amount TQfA as the target fuel injection amountTQf.

In this case, as the target fuel injection amount TQf is determinedaccording to the fresh air intake amount Qac, it is not necessary totake account of the time delay in the change of the intake fresh airamount, and the excess air factor follows the target excess air factorwithout delay.

Next, FIGS. 8A–8E show the change of the output torque of the dieselengine 1 under fuel-based excess air factor control, and FIGS. 9A–9Eshow the change of the output torque of the diesel engine 1 underair-based excess air factor control.

The case where the target excess air factor fluctuates periodically at afixed interval will be considered, as shown in FIGS. 8A and 9A.

In fuel-based excess air factor control, with change of the targetexcess air factor, the target fresh air amount TQa varies as shown inFIG. 8B, and the target fuel injection amount TQf does not vary as shownin FIG. 8C. The change of the target fresh air amount TQa brings about achange of the target opening of the intake throttle 18, as shown in FIG.8D.

In lean burn operation, the output torque of the diesel engine 1 variesdepending on the fuel amount.

In rich burn operation, the output torque of the diesel engine 1 variesdepending on the intake fresh air amount.

Therefore, as shown in FIG. 8B, when the target fresh air amount TQavaries, in a lean burn operation, the output torque does not vary muchas shown by the dashed line of FIG. 8E, but in a rich burn operation,the output torque varies considerably as shown by the solid line of thefigure.

In air-based excess air factor control, the target fresh air amount TQadoes not vary with change of the target excess air factor, as shown inFIG. 9B, therefore the target opening of the intake throttle 18 does notvary either.

On the other hand, the target fuel injection amount TQf does vary, asshown in FIG. 9C.

In lean burn operation, the change of the target fuel injection amountTQf has a large effect on engine output torque, as shown by the dashedline of FIG. 9E, and in a rich environment, it does not vary much asshown by the solid line of the figure.

Now, in the excess air factor control device according to thisinvention, the combustion determination unit 66 and switches 67, 68shown in FIG. 5 apply fuel-based excess air factor control during a leanburn operation, and apply air-based excess air factor control during arich burn operation.

Therefore, the excess air factor can be controlled with sufficientresponse without causing change of engine output torque during a richburn operation or during a lean burn operation.

Referring to FIGS. 10A–10E, if the specific operation requestdetermination unit 41 determines a change-over to NOx reduction fromnormal operation, and the target excess air factor Tlamb varies fromlean to rich at the time t1, in the excess air factor control deviceaccording to this invention, the excess air factor is controlled in arich burn operation by air-based control.

At this time, if the target excess air factor Tlamb varies in stepwisefashion like the target excess air factor basic value Tlamb0 as shown bythe solid line of FIG. 10C, the output torque of the diesel engine 1will change suddenly as shown in FIG. 10E.

However, in the excess air factor control device according to thisinvention, the transient state determination unit 70 shown in FIG. 5sets the transient state flag to unity. Accordingly, the switch 74 setsthe target excess air factor Tlamb to be equal to the value processed bythe delay processing unit 73 until a fixed time has elapsed from thetime t1.

Therefore, the target excess air factor Tlamb0 varies in a stepwisemanner, but the target excess air factor Tlamb varies with a gentleslope, as shown by the broken line of FIG. 10C.

Consequently, variation of the output torque of the diesel engine 1 atthe time t1 is considerably suppressed, as shown by the dashed line ofFIG. 10E.

Moreover, with respect to the target fuel injection timing MITF, theswitch 72, in response to the transient state flag, sets the target fuelinjection timing MITF to be equal to the value processed by the delayprocessing unit 71 until a fixed time has elapsed from the time t1.

Consequently, change of the output torque of the diesel engine 1 at thetime t1 is further suppressed, as shown by the dashed line of FIG. 10E.

Although FIGS. 10A–10E is the case where the target excess air factorTlamb varies from lean to rich, the output torque of the diesel engine 1will change suddenly also if the excess air factor Tlamb changessuddenly when the target excess air factor Tlamb varies from rich tolean.

Also in this case, since delay processing is performed on the targetexcess air factor basic value Tlamb0 to obtain the target excess airfactor Tlamb as well as on the target fuel injection timing basic valueMIT to obtain the target fuel injection timing MITF in response to thetransient state flag of unity, the change of the output torque of thediesel engine 1 is suppressed small.

It is possible to maintain the response of excess air factor control andthat of engine output torque control respectively at desirable levels byappropriately setting the time constant applied by the delay processingunits 71 and 73.

According to this excess air factor control device, therefore,generation of torque shock at the time of switching between lean burnoperation, rich burn operation or stoichiometric air-fuel ratiooperation can be prevented.

The contents of Tokugan 2001-282638, with a filing date of Sep. 18, 2001in Japan, are hereby incorporated by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings.

For example, in the above-described embodiment, the diesel engine 1 isprovided with a variable capacity turbocharger 21 provided with avariable nozzle 24.

However, it is also possible to use a turbocharger provided with amechanism which varies the aperture of a scroll or diffuser instead ofthe variable nozzle 24.

The turbocharger provided with such a regulating mechanism isgenerically termed a variable geometry turbocharger. This invention isapplicable to diesel engines provided with all types of variablegeometry turbocharger.

Furthermore, it is applicable also to a diesel engine provided with acombination of a fixed capacity turbocharger and a waist gate valve.

When using a variable geometry turbocharger, the turbocharger controlunit 49 controls the aperture area of the turbocharger to the areacorresponding to the target fresh air amount TQa.

When using a fixed capacity turbocharger provided with a waist gatevalve, the turbocharger control unit 49 controls the valve opening ofthe waist gate valve to the opening corresponding to the target freshair amount TQa.

INDUSTRIAL FIELD OF APPLICATION

As mentioned above, during lean burn operation of the diesel engine,this invention applies fuel-based excess air factor control, and duringrich burn operation, it applies air-based excess air factor control.

Therefore, in the diesel engine for vehicles which normally performs alean burn operation while performing a rich burn operation for theregeneration of the NOx reduction catalyst, the accuracy and response ofthe excess air factor control is increased while preventing the engineoutput torque from changing.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:

TABLE 1 Pre-mixing Diffusive Regeneration Reduction De- Operationcombustion combustion of DPF of NOx sulphating Air-fuel Lean LeanSlightly Rich Stoichio- ratio lean metric Category Normal NormalSpecific Specific Specific operation operation operation operationoperation Operation A B After de- C Not range sulphating dependentTorque Fuel-based Air-based Fuel-based Air-based Air-based controlcontrol control control control control Excess air λ > 1.4 λ > 1.4 1.4 >λ > 1 λ < 1 λ ≅ 1 factor λ EGR rate Map value Map value Map value Mapvalue 0 x 0.8 x 0.5 x 0.8 Injection 6° retarded Map value 6° advanced15° advanced 10° advanced timing from map from map from map from mapvalue value value value

1. An excess air factor control method for a diesel engine which burns amixture of air supplied by an air supply mechanism and fuel supplied bya fuel supply mechanism, comprising: detecting a running state of thediesel engine; setting a target excess air factor of the mixture basedon the running state; controlling an air supply amount of the air supplymechanism to a target air supply amount calculated from a predeterminedfuel supply amount that has been determined without depending on the airsupply amount of the air supply mechanism and the target excess airfactor when the target excess air factor is larger than a valueequivalent to a stoichiometric air-fuel ratio; and controlling a fuelsupply amount of the fuel supply mechanism to a target fuel supplyamount calculated from a predetermined air supply amount that has beendetermined without depending on the fuel supply amount of the fuelsupply mechanism and the target excess air factor when the target excessair factor is smaller than the value equivalent to the stoichiometricair-fuel ratio.
 2. An excess air factor control device for a dieselengine which burns a mixture of air supplied by an air supply mechanismand fuel supplied by a fuel supply mechanism, comprising: means fordetecting a running state of the diesel engine; means for setting atarget excess air factor of the mixture based on the running state;means for controlling an air supply amount of the air supply mechanismto a target air supply amount calculated from a predetermined fuelsupply amount that has been determined without depending on the airsupply amount of the air supply mechanism and the target excess airfactor when the target excess air factor is larger than a valueequivalent to a stoichiometric air-fuel ratio; and means for controllinga fuel supply amount of the fuel supply mechanism to a target fuelsupply amount calculated from a predetermined air supply amount that hasbeen determined without depending on the fuel supply amount of the fuelsupply mechanism and the target excess air factor when the target excessair factor is smaller than the value equivalent to the stoichiometricair-fuel ratio.
 3. An excess air factor control device for a dieselengine which burns a mixture of air supplied by an air supply mechanismand fuel supplied by a fuel supply mechanism, comprising: a sensor whichdetects a running state of the diesel engine; and a programmablecontroller programmed to: set a target excess air factor of the mixturebased on the running state; control an air supply amount of the airsupply mechanism to a target air supply amount calculated from apredetermined fuel supply amount that has been determined withoutdepending on the air supply amount of the air supply mechanism and thetarget excess air factor when the target excess air factor is largerthan a value equivalent to a stoichiometric air-fuel ratio; and controla fuel supply amount of the fuel supply mechanism to a target fuelsupply amount calculated from a predetermined air supply amount that hasbeen determined without depending on the fuel supply amount of the fuelsupply mechanism and the target excess air factor when the target excessair factor is smaller than the value equivalent to the stoichiometricair-fuel ratio.
 4. The excess air factor control device as defined inclaim 3, wherein the running state detecting sensor further comprises asensor which detects a load of the diesel engine and a sensor whichdetects a rotation speed of the diesel engine, and the controller isfurther programmed, when the target excess air factor is larger than thevalue equivalent to the stoichiometric air-fuel ratio equivalent value,to calculate a target engine torque based on the load of the dieselengine and the rotation speed of the diesel engine, calculate a targetair amount basic value based on the target engine torque and therotation speed of the diesel engine, calculate a conversion coefficientbased on the target excess air factor and the rotation speed of thediesel engine, set the predetermined air supply amount equal to a valueobtained by multiplying the target air amount basic value by theconversion coefficient, and control the air supply amount of the airsupply mechanism to the predetermined air supply amount.
 5. The excessair factor control device as defined in claim 3, wherein the runningstate detecting sensor comprises a sensor which detects a load of thediesel engine and a sensor which detects a rotation speed of the dieselengine, and the controller is further programmed, when the target excessair factor is less than the value equivalent to the stoichiometricair-fuel ratio, to calculate a target engine torque based on the load ofthe diesel engine and the rotation speed of the diesel engine, set apredetermined fuel supply amount equal to a value calculated based onthe target engine torque and the rotation speed of the diesel engine,and control the fuel supply amount of the fuel supply mechanism to thepredetermined fuel supply amount.
 6. The excess air factor controldevice as defined in claim 3, wherein the running state detecting sensorcomprises the controller which is further programmed to count a runningtime of the diesel engine.
 7. The excess air factor control device asdefined in claim 3, wherein the controller is further programmed toapply delay processing to the target excess air factor when the targetexcess air factor varies between a value larger than the valueequivalent to the stoichiometric air-fuel ratio and a value which is notlarger than the value equivalent to the stoichiometric air-fuel ratio.8. The excess air factor control device as defined in claim 3, whereinthe air supply mechanism comprises a variable geometry turbocharger, andthe controller is further programmed to control the air supply amount byvarying a turbocharging pressure of the variable geometry turbocharger.9. The excess air factor control device as defined in claim 3, whereinthe air supply mechanism comprises an exhaust gas recirculation valvewhich recirculates a part of an exhaust gas of the diesel engine intothe air supplied by the air supply mechanism, and the controller isfurther programmed to control the air supply amount by varying anexhaust gas recirculation amount of the exhaust gas recirculation valve.10. The excess air factor control device as defined in claim 3, whereinthe diesel engine comprises an intake passage, the air supply mechanismcomprises an intake throttle provided in the intake passage, and thecontroller is further programmed to control the air supply amount byvarying a throttle opening of the intake throttle.
 11. The excess airfactor control device as defined in claim 1, wherein the diesel enginecomprises a nitrogen oxide trap catalyst which traps nitrogen oxides inan exhaust gas of the diesel engine when an excess air factor of themixture is larger than the value equivalent to the stoichiometricair-fuel ratio, and regenerates a nitrogen oxide trap function of thenitrogen oxide trap catalyst by reducing trapped nitrogen oxides whenthe excess air factor of the mixture is smaller than the valueequivalent to the stoichiometric air-fuel ratio, and the controller isfurther programmed to determine whether or not a regeneration of thenitrogen oxide trap function is required from the running state, set thetarget excess air factor to a value larger than the value equivalent tothe stoichiometric air-fuel ratio when the regeneration is not required,and set the target excess air factor to a value less than the valueequivalent to the stoichiometric air-fuel ratio when the regeneration isrequired.
 12. The excess air factor control device as defined in claim11, wherein the nitrogen oxide trap catalyst accumulates sulfur in theexhaust gas when the target excess air factor is larger than the valueequivalent to the stoichiometric air fuel ratio, and performs adesulphating to release the accumulated sulfur when the target excessair factor is equal to the value equivalent to the stoichiometricair-fuel ratio, and the controller is further programmed to determinewhether or not the desulphating is required from the running state, setthe target excess air factor to a value larger than the value equivalentto the stoichiometric air-fuel ratio when the desulphating is notrequired, and set the target excess air factor substantially equal tothe value equivalent to the stoichiometric air-fuel ratio when thedesulphating is required.
 13. The excess air factor control device asdefined in claim 3, wherein the diesel engine comprises a particulatefilter which traps a particulate matter in the exhaust gas and burns thetrapped particulate matter under a predetermined lean air-fuel ratio,and the controller is further programmed to determine whether or not thetrapped particulate matter is required to be burned, set the targetexcess air factor to a first value larger than the value equivalent tothe stoichiometric air-fuel ratio when the trapped particulate matter isnot required to be burned and set the target excess air factor to asecond value which is larger than the value equivalent to thestoichiometric air-fuel ratio but less than the first value when thetrapped particulate matter is required to be burned.
 14. The excess airfactor control device as defined in claim 13, wherein the controller isfurther programmed to determine that the trapped particulate matter isrequired to be burned when desulphating is required and set the targetexcess air factor to the second value after the diesel engine isoperated with the target excess air factor substantially equal to thevalue equivalent to the stoichiometric air-fuel ratio.
 15. The excessair factor control device as defined in claim 3, wherein the fuel supplymechanism comprises a fuel injector which injects fuel into the dieselengine, and the controller is further programmed to control a fuelinjection timing of the fuel injector to a target injection timing andvary the target injection timing depending on whether or not the targetexcess air factor is larger than the value equivalent to thestoichiometric air-fuel ratio.
 16. The excess air factor control deviceas defined in claim 15, wherein the controller is further programmed toapply delay processing to the target injection timing when the targetexcess air factor varies between a value larger than the valueequivalent to the stoichiometric air-fuel ratio and a value which is notlarger than the value equivalent to the stoichiometric air fuel ratio.17. An excess air factor control device for a diesel engine which burnsa mixture of air supplied by an air supply mechanism and fuel suppliedby a fuel supply mechanism, comprising: a sensor which detects a runningstate of the diesel engine; and a programmable controller programmed to:set a target excess air factor of the mixture based on the runningstate; control an air supply amount of the air supply mechanism to atarget air supply amount calculated from a predetermined fuel supplyamount and the target excess air factor when the target excess airfactor is larger than a value equivalent to a stoichiometric air-fuelratio; and control a fuel supply amount of the fuel supply mechanism toa target fuel supply amount calculated from a predetermined air supplyamount and the target excess air factor when the target excess airfactor is smaller than the value equivalent to the stoichiometricair-fuel ratio; wherein the running state detecting sensor furthercomprises a sensor which detects a load of the diesel engine and asensor which detects a rotation speed of the diesel engine, and thecontroller is further programmed, when the target excess air factor islarger than the value equivalent to the stoichiometric air-fuel ratioequivalent value, to calculate a target engine torque based on the loadof the diesel engine and the rotation speed of the diesel engine,calculate a target air amount basic value based on the target enginetorque and the rotation speed of the diesel engine, calculate aconversion coefficient based on the target excess air factor and therotation speed of the diesel engine, set the predetermined air supplyamount equal to a value obtained by multiplying the target air amountbasic value by the conversion coefficient, and control the air supplyamount of the air supply mechanism to the predetermined air supplyamount.
 18. The excess air factor control device as defined in claim 17,wherein the controller is further programmed, when the target excess airfactor is less than the value equivalent to the stoichiometric air-fuelratio, to calculate a target engine torque based on the load of thediesel engine and the rotation speed of the diesel engine, set apredetermined fuel supply amount equal to a value calculated based onthe target engine torque and the rotation speed of the diesel engine,and control the fuel supply amount of the fuel supply mechanism to thepredetermined fuel supply amount.
 19. The excess air factor controldevice as defined in claim 17, wherein the running state detectingsensor comprises the controller which is further programmed to count arunning time of the diesel engine.
 20. The excess air factor controldevice as defined in claim 17, wherein the controller is furtherprogrammed to apply delay processing to the target excess air factorwhen the target excess air factor varies between a value larger than thevalue equivalent to the stoichiometric air-fuel ratio and a value whichis not larger than the value equivalent to the stoichiometric air-fuelratio.
 21. The excess air factor control device as defined in claim 17,wherein the air supply mechanism comprises a variable geometryturbocharger, and the controller is further programmed to control theair supply amount by varying a turbocharging pressure of the variablegeometry turbocharger.
 22. The excess air factor control device asdefined in claim 17, wherein the air supply mechanism comprises anexhaust gas recirculation valve which recirculates a part of an exhaustgas of the diesel engine into the air supplied by the air supplymechanism, and the controller is further programmed to control the airsupply amount by varying an exhaust gas recirculation amount of theexhaust gas recirculation valve.
 23. The excess air factor controldevice as defined in claim 17, wherein the diesel engine comprises anintake passage, the air supply mechanism comprises an intake throttleprovided in the intake passage, and the controller is further programmedto control the air supply amount by varying a throttle opening of theintake throttle.
 24. The excess air factor control device as defined inclaim 17, wherein the diesel engine comprises a nitrogen oxide trapcatalyst which traps nitrogen oxides in an exhaust gas of the dieselengine when an excess air factor of the mixture is larger than the valueequivalent to the stoichiometric air-fuel ratio, and regenerates anitrogen oxide trap function of the nitrogen oxide trap catalyst byreducing trapped nitrogen oxides when the excess air factor of themixture is smaller than the value equivalent to the stoichiometricair-fuel ratio, and the controller is further programmed to determinewhether or not a regeneration of the nitrogen oxide trap function isrequired from the running state, set the target excess air factor to avalue larger than the value equivalent to the stoichiometric air-fuelratio when the regeneration is not required, and set the target excessair factor to a value less than the value equivalent to thestoichiometric air-fuel ratio when the regeneration is required.
 25. Theexcess air factor control device as defined in claim 24, wherein thenitrogen oxide trap catalyst accumulates sulfur in the exhaust gas whenthe target excess air factor is larger than the value equivalent to thestoichiometric air fuel ratio, and performs a desulphating to releasethe accumulated sulfur when the target excess air factor is equal to thevalue equivalent to the stoichiometric air-fuel ratio, and thecontroller is further programmed to determine whether or not thedesulphating is required from the running state, set the target excessair factor to a value larger than the value equivalent to thestoichiometric air-fuel ratio when the desulphating is not required, andset the target excess air factor substantially equal to the valueequivalent to the stoichiometric air-fuel ratio when the desulphating isrequired.
 26. The excess air factor control device as defined in claim17, wherein the diesel engine comprises a particulate filter which trapsa particulate matter in the exhaust gas and burns the trappedparticulate matter under a predetermined lean air-fuel ratio, and thecontroller is further programmed to determine whether or not the trappedparticulate matter is required to be burned, set the target excess airfactor to a first value larger than the value equivalent to thestoichiometric air-fuel ratio when the trapped particulate matter is notrequired to be burned and set the target excess air factor to a secondvalue which is larger than the value equivalent to the stoichiometricair-fuel ratio but less than the first value when the trappedparticulate matter is required to be burned.
 27. The excess air factorcontrol device as defined in claim 26, wherein the controller is furtherprogrammed to determine that the trapped particulate matter is requiredto be burned when desulphating is required and set the target excess airfactor to the second value after the diesel engine is operated with thetarget excess air factor substantially equal to the value equivalent tothe stoichiometric air-fuel ratio.
 28. The excess air factor controldevice as defined in claim 17, wherein the fuel supply mechanismcomprises a fuel injector which injects fuel into the diesel engine, andthe controller is further programmed to control a fuel injection timingof the fuel injector to a target injection timing and vary the targetinjection timing depending on whether or not the target excess airfactor is larger than the value equivalent to the stoichiometricair-fuel ratio.
 29. The excess air factor control device as defined inclaim 28, wherein the controller is further programmed to apply delayprocessing to the target injection timing when the target excess airfactor varies between a value larger than the value equivalent to thestoichiometric air-fuel ratio and a value which is not larger than thevalue equivalent to the stoichiometric air fuel ratio.